Hemodialysis and ultrafiltration share removal of excess fluid as a primary goal. The amount of fluid to be removed is determined before a treatment session and is related to the patient's pre-treatment weight, fluid addition during treatment and their theoretical dry weight. The amount of fluid to remove and the rate of removal is set on the dialysis or filtration machine and confirmed gravimetrically.
However, it can be difficult to accurately determine a patient's dry weight, which is considered to be the weight that the person would be if their kidneys were properly functioning. What a given patient might weigh if their kidneys were properly functioning is often an unknown variable and can change over time. Yet an accurate determination of the patient's dry weight is important to the successful outcome of a fluid removal session.
Unfortunately, the patient's dry weight is not typically calculated or re-evaluated frequently. Unlike the patient's actual weight, which is measured before and after a fluid removal session, dry weight is often determined much less frequently; e.g. monthly, and much can change in the time between a dry weight determination and a given fluid removal session, which typically occurs three times a week. While being an important variable in fluid removal considerations, dry weight is often difficult to calculate and may vary between sessions.
Errors in fluid volume removal can result in severe hypotension and patient crashing following or during hemodialysis treatment. Sudden and cardiac death (including death from congestive heart failure, myocardial infarction, and sudden death) are common in hemodialysis patients. See Bleyer et al, “Sudden and cardiac death rated in hemodialysis patients,” Kidney International, (1999), 55:1552-1559.
In part to avoid such sudden death, fluid removal rate can be adjusted during a hemodialysis session to prevent too rapid removal or to achieve a specific removal profile. The removal rate is controlled by adjusting pump parameters, and the amount of fluid removed is confirmed gravimetrically (i.e., by weighing). However, current standard-of-care hemodialysis does not include any monitoring of fluid volume within the tissue to get an accurate reading of actual fluid status. Some suggestions have been made to monitor hematocrit levels during dialysis to monitor blood volume in an attempt to avoid the potentially dire consequences of fluid imbalance. However, monitoring blood fluid volume alone may not present as accurate of a picture as monitoring the ratio of blood to tissue volume and ensuring the ratio stays within predefined parameters during hemodialysis for purposes of patient safety.